The present invention relates to X-ray computerized tomography (hereinafter abbreviated to "CT") apparatus, and particularly to an X-ray CT apparatus for use preferably for cardiac function diagnosis, a method for controlling the apparatus and a method for generating images by using the apparatus.
An X-ray CT apparatus is provided, as shown in FIG. 4, with a scanner gantry section 10 for performing radiation and detection of R-rays, a data processing section 20 for processing measurement data detected by the scanner gantry section 10 into a CT image signal, and a display unit 30 for displaying the CT image. The scanner gantry section 10 is provided with a rotary disc 11, an opening portion 14 formed in the rotary disc 11, an X-ray tube 12 mounted on the rotary disc 11, an X-ray detector 15 attached so as to face the X-ray tube 12, and a detector circuit 16 for converting an output signal of the X-ray detector 15 into a digital signal S1. The rotary disc 11 is designed so as to rotate around an object 18 laid down on a bed (not shown) disposed in the opening portion 14 while the X-rays are radiated onto one sliced section of the object 18. As a result of one scan, one slice image can be obtained by one scan. If such a scan is repeated by a plurality of times while the position of the rotary disc 11 relative to the object 18 is being changed, data of a plurality of images can be obtained with respect to a desired image pick-up portion. The rotation of the rotary disc 11 and the width of X-ray flux are controlled by a scanner gantry control circuit 17.
As the scan method for obtaining CT images, there are a full scan method in which one scan is completed by making an X-ray source rotate around an object by 360 degrees and a half scan method in which scan is completed by X-ray movement by about 210.about.240 degrees. In the full scan method, generally, image pick up is carried out in a time of one to several seconds per scan.
In the data processing section 20, on the other hand, a digital signal S1 is sent to a CT image operation section 21 from the detector circuit 16 at every scan. Operation such as arrangement processing, filtering processing, reverse projection processing, etc. are performed in the CT image operation section 21, so that image data are generated with respective to the sliced section. Attribute information relating to the images is added to the image data in an image information adding section 22. The image data are processed into a display signal in a display circuit section 24. Slice images are displayed on a display unit 30.
There is also such a CT apparatus in which a three-dimensional image generating section is provided (between the image information adding section 22 and the display circuit section 24) in the data processing section so that three-dimensional information is extracted from a series of CT images to thereby display three-dimensional CT images. In this case, data of a plurality of tomography images obtained by a plurality of scans are reconstituted into three-dimensional image data.
Two-dimensional sectional images in the direction of a body axis of the object crossing the sliced face in the scanning direction, such as sagittal images or coronal images, can be obtained in a manner in which only data components of a specific plane parallel with a body axis are extracted out of three-dimensional image data, and two-dimensional images of the plane are reconstructed on the basis of the extracted data components. Three-dimensional images can be obtained in a manner in which data components of specific three-dimensional coordinates are extracted out of three-dimensional image data, and three-dimensional images are reconstructed on the basis of the extracted data components.
The sagittal images, coronal images and three-dimensional tomography images can be obtained by means of computer graphics technique disclosed in the "Three Dimensional Processing For Computer Tomography Images" described in the magazine titled "PIXEL", Vol. 16, pp. 28-35, 1984, 1.
In a case of performing cardiac image-diagnosis by using such an X-ray CT apparatus, image distortion due to cardiac pulsation occurs in the CT images. Particularly in the case of a plurality of CT images which are different in measurement time from each other, the images are not coincident with each other in pulsation phase so that the image distortion increases in the three-dimensional images or in the reconstructed sagittal (or coronal) images to lose values of diagnosis. In order to avoid this disadvantage, there has been proposed a method (ECG gated CT image reconstituting method) in which image operation is carried out by collecting only data measured from the measurement data by a plurality of scans at a predetermined phase of the electrocardiographic complex. In this method, however, since only specific measurement data are used among a large number of measurement data, there is a disadvantage that image noise due to shortage of the quantity of data becomes large or the time for test and the quantity of use of a contrast medium increase because the number of scans is increased so as to make the SN ratio large.
In the conventional X-ray CT apparatus, the rotational speed of the rotary disc, that is, the scan speed has no connection with the period of the cardiac pulsation of a object and scan is performed at a constant speed. Accordingly, among a plurality of slice images which are different in measurement time from each other, the images are not coincident in phase of pulsation with each other so that distortion appears in cardiac sagittal or coronal images or cardiac three-dimensional images to make the diagnosis difficult.